Pressure sensor for a baghouse

ABSTRACT

A pressure transducer, particularly a transducer for measuring gaseous pressure differential across a chamber wall is disclosed. The transducer finds particular application in an air cleaning system. The pressure transducer of the invention is designed and arranged to include a load cell disposed between the chamber wall and a diaphragm which is designed and arranged to seal about a hole in the chamber wall.

This application is a continuation of application Ser. No. 08/354,538filed Dec. 13, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to pressure transducers and in particular to atransducer for measuring gaseous pressure differential across a chamberwall. The pressure transducer described has particular application inair cleaning systems.

2. Description of Prior Art

A common form of air cleaning system forces dust-laden air throughfilter bags fabricated from a material such that dust intended to beremoved builds up on one surface of each bag. A multiplicity of bags maybe used together in parallel and suspended from apertures in a dividingwall or cell plate within a purpose-designed vessel. Such a system isknown as a "baghouse".

It is known to monitor the pressure differential across the filter bagsto provide an indication of the state of the filter bags and thusinitiate manual or automatic bag cleaning. The pressure differentialarises due to frictional and kinetic energy losses through each bag.Conventional systems use a water manometer with sensing tubes connectedinto the vessel either side of the cell plate. Electronic pressuretransducers have also been proposed but, as with the U-tube manometer,require coupling hoses and sensing ports open to the interior of thecell.

These conventional pressure indicators suffer from the disadvantage thatthe transducer connection on the dusty air side of the cell fills withcontaminants to cause erroneous pressure differential readings. Cloggedmanometer ports either give a false indication of dust build-up on thefilter bags, or cause early indications of filter problems to be missed.

3. Identification of Object of the Invention

It is therefore an object of the present invention to provide a gaseouspressure transducer which will at least go some way towards overcomingthe above mentioned disadvantage or which will at least provide theindustry with a useful choice.

SUMMARY OF THE INVENTION

Accordingly, in one aspect the invention may broadly be said to consistin a pressure transducer for measuring gaseous pressure differentialacross a wall having an electrical output which changes as a function ofsaid pressure differential comprising:

a diaphragm adapted to be sealably mounted over an aperture in the wall,

a load cell having first and second load points, with the first loadpoint of the load cell fixed to the diaphragm, and coupling means whichrigidly connect the second load point of the load cell to the wall,

the output nodes of said load cell forming the output nodes of thetransducer.

Preferably the diaphragm is shaped as a conical section with aperforated plate attached at its periphery to the periphery of thediaphragm, and the load cell is disposed between said diaphragm and saidplate.

In a second aspect, the invention may broadly be said to consist in anair cleaning system comprising a vessel having a dividing wall whichforms two chambers within the vessel, with the dividing wall includingone or more apertures therethrough, one or more dust filter bagssealably suspended from a corresponding dividing wall aperture and apressure transducer for measuring the pressure differential across thedividing wall to provide an electrical output which changes as afunction of the pressure differential,

with the pressure transducer in turn comprising;

a diaphragm adapted to be sealably mounted over an aperture in the wall,

a load cell having first and second load points, with the first loadpoint of the load cell fixed to said diaphragm, and coupling means whichrigidly connect the second load point of the load cell to the wall,

where the output nodes of the load cell form the output nodes of thetransducer.

BRIEF DESCRIPTION OF DRAWING

The preferred form of the present invention will now be described withreference to the accompanying drawing in which:

FIG. 1 shows a diagrammatic representation of a dust extraction systemand associated pressure differential monitoring; and

FIG. 2 is a cross-section of a pressure transducer according to thepresent invention.

DESCRIPTION OF THE INVENTION

FIG. 1 shows a baghouse dust extraction system for dust-laden air withwhich the pressure transducer of the present invention is used. A cellor vessel 1 having an outer wall receives dust-laden air through port 2and discharges clean air through port 3. Dust entrained within theincoming air is filtered out by being passed through a number of filterbags which share the total air flow. The filter bags are suspended froman inner transverse cell plate 5 which divides cell 1 into a dusty airside 6 and a clean air side 7. Filter bags 4 are suspended fromcorresponding apertures 8 uniformly dispersed across cell plate 5. Airpasses through the filter bags leaving dust on the exterior surface ofthe bags and the clean air passes up through apertures 8 to be extractedthrough port 3.

A pressure transducer 9 monitors the pressure differential across cellplate 5 and then across the parallel arrangement of filter bags. As thedust on the filter bags 4 builds up, so too does the pressuredifferential across cell plate 5. This is monitored by pressuretransducer 9 and a signal from the pressure transducer drives a digitaldisplay 10 which indicates pressure drop and may also drive a chartrecorder 11 and/or an alarm 12. The analogue output 13 from the pressuretransducer may be used to control a filter bag cleaning system such aspulsed compressed air jets directed into the filter bags from the top ofcell 1.

The construction of pressure transducer 9 is shown in FIG. 2. It isshown mounted in an aperture 14 in cell plate 5. The primary sensingelement of the transducer is a load cell 15 which, when provided with asuitable electrical input signal, provides an electrical output via leadconductors 16 representative of the force applied to the load points ofthe load cell. The load cell is mounted rigidly to cell plate 5 throughload point 17 which is fixed to a perforated cover 18. Perforations orholes 30 are provided in plate 18. Cover 18 extends beyond the peripheryof aperture 14 and is bolted or otherwise fixed to the cell plate acrossaperture 14.

A flexible diaphragm 19, which may be made for example from rubber or aplastics material, is fixed to transducer cover 18 about its periphery20 and in sealing engagement with the rim of aperture 14. Cover 18 ispreferably bonded with silicone to the periphery 20 of diaphragm 19.Diaphragm 19 thus is subject to the same pressure differential as iscell plate 5. This is because cover 18 is perforated with holes 30,allowing the upper surface of diaphragm 19 to be subject to the samepressure as applies to the clean air side of cell plate 5.

Diaphragm 19 comprises a concial portion 21 and a base portion 22. Baseportion 22 is fixed to the second load point 23 of load cell 15. Thus aforce F applied to diaphragm 19 in the direction of the arrow will betransmitted to load cell load point 23, and because load point 17 isfixed, load cell 15 provides an electrical output proportional to forceF. In the application described, force F is produced by the pressuredifferential between the dusty air and clean air sides of cell plate 5.The output of load cell 15 is therefore proportional to the pressuredifferential across the filter bags 4.

The leads 16 may run through via one of the holes 30 to a terminalenclosure (not illustrated) mounted on plate 18.

A cantilever type load cell has been shown, but other configurationscould be used to provide similar results. It is preferred that the loadcell be physically small since there are advantages in providing apressure transducer which is of relatively small total size. The loadcell should have a capacity less than 5 kg in order to providesufficient accuracy at the pressure differentials normally experiencedin the application described. An example of a suitable load cell is aTanaka model C2G1 series load cell manufactured by EMC. It will beappreciated that this load cell is given by way of example only.

The present transducer has the advantage that no tubes or ports arerequired which can be contaminated. The diaphragm protects thetransducer from dust as a natural consequence of its structure andfunction and the perforations in cover 18 may be provided with filtersto prevent what little dust may enter the clean air plenum from enteringthe interior of the transducer. The only connections required betweenthe transducer and the external measuring system are the electricalconductors from the load cell.

What is claimed is:
 1. A transducer for measuring pressure differentialcomprising,a cover member (18) having an outer annular portion arrangedand designed for sealable securement beyond the periphery of a hole in awall, where differential fluid pressure exists on opposite sides of awall, a diaphragm (19) sealed about and carried by a portion of saidcover member (18), said diaphragm having outer and inner surfaces andarranged and designed such that said outer surface may be exposed onlyto fluid pressure on one side of a wall, a load cell (15) having first(17) and second (23) load points, said first load point (17) secured tosaid cover member (18), said second load point (23) secured to saiddiaphragm (19), a fluid passage (18) in said cover member (18) arrangedand designed so that fluid pressure on an opposite side of a wall may beapplied to said inner surface of said diaphragm, but not to said outersurface of said diaphragm, said load cell having output leads (18) forproducing an electrical output signal which changes as a function ofpressure differential between said inner and outer surfaces of saiddiaphragm.
 2. The transducer of claim 1 wherein,said cover member (18)is a plate, and said plate is perforated so that one or more passagesexist from outside said plate to its inside so that fluid pressure maybe applied to said inner surface of said diaphragm.
 3. The transducer ofclaim 1 wherein said diaphragm (19) is sealed and carried by saidannular portion of said cover member (18).
 4. The transducer of claim 7wherein said diaphragm (19) includes a conical section having an outerannular portion which is sealed to and carried by said annular portionof said cover member.
 5. In an air cleaning system having a vessel witha lower inlet chamber for dust entrained air and an upper outlet chamberfor relatively dust-free air, an inner transverse wall between saidchambers having a plurality of apertures therein, and a plurality offilter bags suspended from said transverse wall and received within saidapertures; an improved pressure transducer for measuring fluid pressuredifferential between said lower and upper chambers comprising:adiaphragm having inner and outer sides and designed and arranged to besealably mounted within an opening of a wall of said lower inlet chamberand having its inner side in gaseous communication with said lower inletchamber, said diaphragm being secured and sealed at its outer peripherywith respect to said wall; a plate across said opening and across saiddiaphragm and secured at its outer periphery to said wall; a load cellhaving first and second load points, said first load point being securedto said diaphragm and said second load point being secured to saidplate; a fluid passage for communication of fluid pressure to said outerside of said diaphragm from said upper outlet chamber, and output leadsconnected to said load cell for producing an electrical output signalwhich changes as a function of gaseous pressure differential betweensaid lower and upper chambers.
 6. In an air cleaning system as set forthin claim 5 wherein:said pressure transducer is mounted within an openingof said inner transverse wall, and said fluid passage is an opening insaid plate which provides gaseous communication between said upperoutlet chamber for clean filtered air and said outer side of saiddiaphragm which faces dust entrained air.